3GPP LTE (3rd generation partnership projecting long term evolution) communication system is schematically described for one example of a wireless communication system to which the present invention is applicable.
FIG. 1 is a schematic diagram of E-UMTS network structure as an example of a wireless communication system. E-UMTS (evolved universal mobile telecommunications system) is the system evolved from a conventional UMTS (universal mobile telecommunications system) and its basic standardization is progressing by 3GPP. Generally, E-UMTS can be called LTE (long term evolution) system. For the details of the technical specifications of UMTS and E-UMTS, Release 7 and Release 8 of ‘3rd Generation Partnership Project: Technical Specification Group Radio Access Network’ can be referred to.
Referring to FIG. 1, E-UMTS consists of a user equipment (UE) 120, base stations (eNode B: eNB) 110a and 110b and an access gateway (AG) provided to an end terminal of a network (E-UTRAN) to be connected to an external network. The base station is able to simultaneously transmit multi-data stream for a broadcast service, a multicast service and/or a unicast service.
At least one or more cells exist in one base station. The cell is set to one of bandwidths including 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz and the like and then provides an uplink or downlink transmission service to a plurality of user equipments. Different cells can be set to provide different bandwidths, respectively. A base station controls data transmissions and receptions for a plurality of user equipments. A base station sends downlink scheduling information on downlink (DL) data to inform a corresponding user equipment of time/frequency region for transmitting data to the corresponding user equipment, coding, data size, HARQ (hybrid automatic repeat and request) relevant information and the like. And, the base station sends uplink scheduling information on uplink (UL) data to a corresponding user equipment to inform the corresponding user equipment of time/frequency region available for the corresponding user equipment, coding, data size, HARQ relevant information and the like. An interface for a user traffic transmission or a control traffic transmission is usable between base stations. A core network (CN) can consist of an AG, a network node for user registration of a user equipment and the like. The AG manages mobility of the user equipment by a unit of TA (tracking area) including a plurality of cells.
The wireless communication technology has been developed up to LTE based on WCDMA but the demands and expectations of users and service providers are continuously rising. Since other radio access technologies keep being developed, new technological evolution is requested to become competitive in the future. To this end, reduction of cost per bit, service availability increase, flexible frequency band use, simple-structure and open interface, reasonable power consumption of user equipment and the like are required.
Thus, the increase of data traffic consumed through a recent LTE service due to the development of the wireless communication technology has been pointed out as one problem. Hence, required is a method of expanding data capacity, which can be provided by a network without massive facility investments while satisfying the demand for client's data use. In this context, there is an ongoing movement for off-loading wireless data traffic concentrated on 3G or LTE network using the wireless LAN communication system available on an unlicensed band such as ISM (Industrial, Scientific and Medical) band. The concept of the unlicensed band is described as follows.
First of all, unlicensed bands or license-free spectrums mean the radio spectrums available for everyone without licenses. For instance, the unlicensed bands normally used in U.S.A. mean the bands of 900 MHz, 2.4 GHz, 5.2/5.3/5.8 GHz, 24 GHz, 69 GHz or higher. Other countries may have different frequency bands according to the rules for the unlicensed bands. For example, the unlicensed bands may include ISM (Industrial, Scientific and Medical) band and TVWS (Television White Space) band. In addition, previously unused radio spectrums tend to become unlicensed bands owing to various technological reasons or technical changes. Since a frequency of such an unlicensed band is higher than frequencies used by most of licensed bands and a transmission power of a base station is relatively low, the unlicensed band may be appropriate for the implementation of a small cell capable of servicing an area in small size effectively. Moreover, since a communication service provider does not secure an exclusive frequency use right, an unlicensed band is advantageous in sharing a considerable level of capacity with a low cost incomparable with that of a network establishment on a licensed band.
However, it is difficult for an unlicensed band to secure a communication quality on a level that can be provided by a communication service on a licensed band. Moreover, although the standardization for the linkage between HSPA or LTE service corresponding to 3GPP standard technology and Wi-Fi corresponding to IEEE standard technology is in progress, it is disadvantageous in having difficulty in providing a perfectly integrated service in aspects of handover, QoS guarantee and the like. To solve these problems, the ongoing discussion about LTE technology (LTE-U: LTE on Unlicensed Spectrum or U-LTE) on an unlicensed band is in progress.